'Mr Rylance, Mr McBurney, Ms Margolyes, Mr Hickey – to the stage, please." There are four flights of stairs from the dressing room to the stage at the Duchess, one of the few West End theatres still unmodernised since the 1930s. Everywhere smells damp – appropriately desolate when you're performing Beckett's Endgame. I take the stairs two at a time.

Most nights, I'm not nervous. Tonight I am. Think of something else. Paragliding in the Alps. Now that was frightening: spreading out the parachute behind you on a 45-degree slope ending in a sheer drop. I recall shivering, trying to listen to the instructor.

"Are you cold?" I shake my head, but the wardrobe supervisor hangs a coat around my shoulders anyway. "Stand by." I walk on to the stage. The curtain is a thin gauze. The audience can't see me but I can see them. I fix my gaze on the chair where Mark Rylance, playing Hamm, is hidden under a plastic sheet.

I am Clov. No, I am not. I am still thinking about the chatter of the audience. I block it out, but the internal chattering continues. Clov. Nearly clou in French, which means nail, just as Nagg, Hamm's father, means nail in German. And Nell, his mother, sounds like nail. Three nails. There were three nails on the cross. Quiet, for God's sake, I tell myself. None of this helps. I shift my feet, clear my throat. Under the sheet, Mark clears his throat in reply. I glance once more at the audience. I wonder how many bought seats simply because they want to see a West End Show. If so, they are in for a shock.

Ultimately, theatre takes place in the minds of the audience: they all imagine the same thing at the same time. But Beckett is special, Endgame particularly so. It is unlike anything else I have played: fastidiously specific, utterly elusive. At any one moment in the performance, you will be aware of someone laughing hysterically, another weeping, while others sit silent, astounded or baffled. Endgame resists narrative and even thematic explanation. How you play it has to reflect this. If you decide something too much in advance, you forget the element that gives the play life – the audience.

My opening section consists entirely of moving ladders and folding sheets. A few giggles. A matinee audience. Then I am off and waiting for my next entrance. Mark whistles and I plunge on to the stage by this side. I know that in three lines' time we'll be at the part where they laughed like drains last night. But right now I have to be here, forgetting last night. How do you forget? You can't. But you can let go.

"Let go!" the paragliding instructor yells at me as we run forward. In a few moments, I know the parachute will fill with air. Fuck, I am going to die. I pull and pull and seem to be going nowhere. My mouth is dry. Mark suddenly barks a line. Blimey: never heard that before. Should I be looking at him or at the audience? Last night, it felt wrong looking at him. I'll look out and then look back. I can hear the audience react. Good: right choice.

Hamm: "Go and get the sheet!"

I do not react. Hamm: "CLOV!"

Clov: "Yes."

Hamm: "I'll give you nothing more to eat."

Clov: "Then we'll die."

Good. That felt blank. Let's see what happens with Hamm's next line.

Hamm: "I'll give you just enough to keep you from dying – you'll be hungry all the time."

The audience chuckle as one. Then there's more laughter. Suddenly I am airborne, not thinking about the words but held up by them, floating on them. Like navigating the eddies and air currents above that Alpine landscape. Playing Beckett is like playing music: words as a score. He marks speeches with dynamic terms – violently, tonelessly, sadly – and meticulously places groans, yawns, laughs and mutterings. Omit these and the meaning changes completely.

Hamm: "Clov?"

Clov: "Yes."

Hamm: "You don't think we are beginning . . . to mean something?"

Laughter. Good. But last night there was silence at that point. In fact, there was silence most of the way through, then wild applause at the end. Strange.

Everyone sees something different in Endgame: a biblical apocalypse, a portrait of painful co-dependency, a confession of guilt and dignity in the face of death, a night of baffling hopelessness, a meaningless babble. Each interpretation reveals an absurd truth – not about the play, but about the person watching it.

The moment you think it's about something, Beckett reminds us that we are simply in the theatre. "I'm warming up for my last soliloquy," Hamm yells at one poignant climax. Beckett refuses to soothe us with reassuring sense. Night after night, you feel the meaning shift as each audience reacts to something different. That's what makes it so hard and so rewarding to play.

Clov: "I'm leaving you."

Hamm: "No!"

Clov: "What's there to keep me here?"

Hamm: "The dialogue."

Shit, they laughed there last night. I squirm in annoyance. Shut up – stop thinking of the audience.

But you can't with Beckett. It's like trying to stop thinking of the ground beneath you when you are 2,000ft up in the air, watching a landscape spread out beneath you. One false move and nothing means anything. So you must be ready to react, instantly, at every moment. You just have to be there. Forget, be present – and let it happen.